This invention is directed to a novel class of crosslinked anion exchange resins and their method of preparation.
Ion exchange resins have heretofore been conventionally manufactured by aqueous suspension polymerization of a monomer mixture comprising, for example, a large proportion of a monoethylenically unsaturated monomer and, as a crosslinker, a small proportion of a polyethylenically unsaturated monomer to form an intermediate crosslinked copolymer. The intermediate crosslinked copolymer resulting from the suspension polymerization is thereafter treated to introduce either a cationic or anionic ion exchange functional group by methods well known in this art. Unfortunately, this aqueous suspension polymerization which utilizes mechanical agitation to form the monomer droplets from which the resin heads are formed, yields crosslinked copolymer particles having a wide range of particle sizes which must be separated by screening or the like in order to furnish a product of relatively uniform size. The fines and oversize beads screened from the product often represent a substantial manufacturing loss as they have little economic value. Once crosslinked, the off-size beads cannot be reused as starting materials for suspension polymerization or recycled in any useful ion exchange process.
With anion exchange resins, crosslinking of the polymer bead prior to functionalization has generally been thought essential heretofore, in view of the requirements of the functionalization reactions. For example, uncrosslinked polymers tend to dissolve in the solvents necessary for swelling of the beads, a step generally required prior to amination of the resin. Accordingly, crosslinking of ion exchange resin copolymer precursors has been considered essential where the bead is to be subjected subsequently to vigorous chemical reaction to form functional ion exchange sites within the polymer backbone.